Al-mg-zn-alloy having high stress corrosion resistance



United States Patent Al-Mg-Zn-ALLOY HAVING HIGH STRESS CORROSIDN RESISTANCE No Drawing. Application January 23, 1956 Serial No. 560,890

8 Claims. (Cl. 75-141) assignor to Meinerzhagen, West- The present invention relates to an aluminum alloy and workpiece, and more particularly it relates to an aluminum alloy and workpiece having high stress corrosion resistance.

It has long been attempted to form aluminum alloys with zinc and magnesium which have the same resistance to corrosion under stress as aluminum alloys formed with copper and magnesium. Attempts have been made to add to an aluminum, zinc and magnesium alloy small quantities of other alloying elements such as chromium, vanadium or copper in order to obtain an alloy of stress corrosion characteristics similar to those of the aluminum, copper, magnesium alloys. Practical experience with these different types of aluminum alloys has however shown that up to now the stress corrosion characteristics of aluminum alloys of the type aluminum, zinc, magnesium are inferior to those of the aluminum alloys of the type aluminum, copper, magnesium, even when chromium, vanadium or small quantities of copper areadded to the zinc type alloy.

It is therefore an object of the present invention to provide an aluminum alloy of the aluminum, zinc, magnesiurn type which has great resistance against stress corrosion.

It is another object of the present invention to provide an aluminum alloy of the aluminum, zinc, magnesium type which has stress corrosion characteristics comparable to those of aluminum, copper, magnesium alloys.

It is a further object of the present invention to provide a method of forming a workpiece having highstress corrosion resistance from an aluminum alloy of the aluminum, zinc, magnesium type.

It is yet another object of the present invention to provide a workpiece having high stress corrosion resistance made from an aluminum alloy of the aluminum, zinc, magnesium type.

Other objects and advantages of the present invention will become apparent from a further reading of the description and the appended claims.

With the above objects in view, the present invention comprises an aluminum alloy consisting essentially of commercial aluminum and of between 4% and 12% Zinc, between 1.5% and 6% magnesium, between 0.1% and 1.0% silver, and an amount of at least one additional metal being selected from the group consisting of between 0.1% and 0.6% chromium, between 0.1% and 1.5% manganese, and between 0.03% and 0.15% vanadium.

The present invention also includes in a method of forming a workpiece having high stress corrosion resistance, the steps of forming an alloy consisting essentially of commercial aluminum and of between 4% and 12% zinc, between 1.5% and 6% magnesium, between 0.1% and 1.0% silver, and an amount of at least one additional metal being selected from the group consisting of between 0.1% and 0.6% chromium, between 0.1% and 1.5% manganese, and between 0.03% and 0.15% vanadium, and mechanically working said alloy so as to form a workpiece of predetermined shape.

The present invention also contemplates a workpiece made by mechanically working an aluminum alloy consisting essentially of commercial aluminum and of bet 2,823,994 Patented Feb. .18, 1958 tween 4% and 12% zinc, between 1.5% and 6% nesium, between 0.1% and 1.0% silver, and an amount of at least one additional metal being selected from the group consisting of between 0.1% and 0.6% chromium, between 0.1% and 1.5 manganese, and between 0.03% and 0.15 vanadium.

Previously some times the opinion has been voiced that the stress corrosion characteristics of aluminum alloys of the aluminum, zinc, magnesium type could be improved and made equivalent to the stress corrosion characteristics of aluminum alloys of the type aluminum, copper, magnesium, by adding to the zinc type aluminum alloys certain alloy elements such as chromium, vanadium or copper. While the addition of these alloying metals tends to improve the stress corrosion resistance of workpieces, made of aluminum alloys of the type aluminum zinc, magnesium by mechanical working, many years of practical experience with both types of alloys have shown that equivalent characteristics with respect to intercrystalline and stress corrosion cannot be obtained. While workpieces of the alloy type aluminum, copper, magnesium remain free of tension and stress cracks, workpieces made of aluminum alloys of the type aluminum, zinc, magnesium and containing small quantities of alloying metals such as chromium or copper show frequently stress cracks, especially in areas of three dimensional deformation. These stress cracks occurring in the areas of three dimensional deformation of workpieces made of alloys of the aluminum, zinc, magnesium type with addition of small quantities of chromium and/or copper are found in areas which obviously have been deformed and also in areas where unknowingly three dimensional deformation had taken place. i

The erroneous assumption that aluminum alloys of the aluminum, zinc, magnesium type could be improved in stress corrosion resistance by the addition of small quanti ties of chromium, vanadium or copper so that the zinc type alloy then would be equivalent to the copper type aluminum magnesium alloy can be traced to laboratory test results which do not accurately reflect practical working conditions. It is customary to examine the stress corrosion resistance of extrusion press shapes and die-pressings by the so-called fork-test in sodium chloride-containing or similar solutions. This quick testing method does not permit to draw accurate conclusions as to the actual stress corrosion characteristics of the workpiece under practical working conditions.

The reasons for the unreliability of this test may be found in the fact that the pH value of the corrosion test medium changes continuously during the testing period. While at the beginning of the test the pH of the testing solution lies in the acidic range, it will change during the testing and dependent on the relationship between the quantity and concentration of the solution and the entire surface area of the immersed workpiece, more or less quickly towards the neutral point and eventually even into the alkaline range. However, stress corrosion cracks occur in a practically considerable degree only in acidic corrosion media such as are also represented by the atmosphere which nearly exclusively has to be considered as the corrosion medium for practical purposes.

The conditions with respect to the potential difference between the mixed crystal and the grain-boundary which are decisive for stress-corrosion behaviour, are diametrically opposite with respect to the compared alloys, namely aluminum alloys of the aluminum, copper, magnesium type and of the aluminum, zinc, magnesium type. While in the case of aluminum, copper, magnesium alloys the grain-boundary is nobler than the mixedcrystal, in the case of aluminum, zinc, magnesium alloys the mixed crystal is nobler than the grain-boundary. Since always the less noble constituent is electrochemically dissolved magand since the stress corrosion cracks of the aluminum alloys always are formed along the grain-boundaries, it follows that the aluminum, zinc, magnesium alloys fundamentally tend to stress-corrosion while the aluminum,

' copper, magnesium alloys do not. In view of this electrochemical difference between the two types of aluminum alloys one has attempted to keep the potential difference between the mixed crystal and the grain-boundary in the case of aluminum alloys of the aluminum, zinc, magnesium type as small as possible, so far as this could be achieved by alloying and finishing measures. The relative potential difference between the mixed crystals and the grain-boundaries will lead by the addition of nobler alloying elements to an enobling of the grain-boundaries, since the grain-boundaries of the casting are enriched with alloying costituents due to the presence of residual melt, and also since grain-boundaries which originate by the deformation and recrystallization are enriched with alloying constituents due to the improved diffusion possibilities in these areas as compared with areas in which the crystal structure has not been disturbed.

During hot deformation of alloys of the type aluminum, zinc, magnesium spontaneously crystallization takes place, and this recrystallized structure causes extraordinary susceptibility of the material against intercrystalline and stress-corrosion. It has therefore been tried to counteract this recrystallization tendency during hot deformation and subsequent solution heat treatment by alloying measures such as addition of chromium and manganese. How ever, prevention of spontaneous recrystallization of the surface areas which are exposed to the atmosphere and which are the starting points of stress-corrosion attacks could not be achieved under actual working conditions with the so far proposed alloying measures.

It is in the nature of the fork-test which has been discussed above that the exposed area of the fork does not possess the same micro structure as the surface area of the workpiece which in practice is primarily exposed to corrosion attack. This surface area or skin tends in the case of both types of aluminum alloys to recrystallization. Due to the differences between grain-boundaries and mixed crystals of the two types of aluminum alloys which differences were discussed above, workpieces made of aluminum alloys of the type aluminum, copper, magnesium possess in their corrosion exposed surface areas the best structure, while workpieces made of alloys of the type aluminum, zinc, magnesium show the most unfavorable structure in their surface areas. In the exposed areas of the test-fork however, conditions are reversed. Consequently, the fork-test indicates best results with alloys of the type aluminum, zinc, magnesium, and most unfavorable results with alloys of the type aluminum, copper, magnesium, while in practical usage of workpieces made of these two types of aluminum alloys, better results are obtained with those made of aluminum, copper, magnesium alloys than with those made of aluminum, zinc, magnesium alloys.

The above described discrepancies and contradictory results can be eliminated by a testing method combining bend tests with exposure to atmospheric conditions, as

will be hereinafter'described. T-profiles of the various alloys which are produced with extrusion presses or by machining, are bent in the plane of the T-flange about the stem and then exposed to real or simulated atmospheric conditions. After more or less prolonged testing periods, typical stress-corrosion cracks are formed in the once upset areas of the T-profile stem. Comparison tests performed according to this method with workpieces made of aluminum alloys of the type aluminum, copper,'magnesium and of the type aluminum, zinc, magnesium of so far known composition show that there is no equivalence in the resistance -against intercrystalline and stress corrosion between these two known types of aluminum alloys.

Surprisingly it hasnow been found thatthe resistance of workpieces produced by mechanical working from aluminum alloys of the type aluminum, zinc, magnesium, against intercrystalline and stress corrosion can be considerably improved by including in these alloys in addition to chromium, and/ or manganese, and/ or vanadium, and possibly also copper, in accordance with the present invention also a relatively small amount of silver. A combined alloying addition of chromium and silver has proven to give especially advantageous results according to the present invention. In view of the great accuracy of the used testing method which is evidenced in the experimental results indicated below, it can be stated that the suspectibility of workpieces made of aluminum alloys of the type aluminum, zinc, magnesium against intercrystalline and stress corrosion can be practically eliminated by combined alloying additions of either chromium and silver, or manganese and silver, or vanadium and silver. According to a preferred embodiment of the present invention excellent results are obtained by further including also a small percentage of copper. According to the present invention workpieces of high mechanical stress resistance and high resistance against intercrystalline and stress corrosions are produced by mechanical working of aluminum alloys of the following composition:

TABLE I Zinc a a Between a% and 12%. Magnesium Between 1.5% and 6%. Silver Between 0.1% and 1.0%.

At least one of the following elements: Chromium, manganese, vanadium, in the indicated percentage amounts Chromium 2. Between 0.1% and 0.6%. Manganese Between 0.1% and 1.5%. Vanadium Between 0.03% and 0.15%.

TABLE II Composition of Test Alloys Alloy No.

Zn, Mg, Cu, Ag, Cr, Fe, Si, Al, perperperperperperperpercent cent cent cent cent cent cent cent;

0.02 0.29 0.11 Balance. 3.85 0.32 0.12 Do. 3.89 0.28 0.13 Do.

4. 0.18 0.41 0.23 0.10 Do. 4. 1.52 0.40 0.29 0.12 Do. 4. 0.04 0.- 0.40 0.27 0.16 Do.

9. 0.16 0.32 0.24 0.10 Do. 9. 0.04 0.35 0.30 0.27 010 Do. 9 9.20 1.99 0.02 0.74 0.27 0.22 0.12 Do.

TABLE III Life Span in Days Alloy No.

1 2 3 4 5 Average The comparison of alloy No. 1 with alloys No. 2 and 3 in Table III shows that aluminum alloys of the type aluminum, zinc, magnesium and practically free of additional alloying metals are far inferior to alloys of the type aluminum, copper, magnesium.

A further comparison of the alloys No. 4 and 5 with alloy No. 6 shows that aluminum alloys of the type aluminum, zinc, magnesium containing about 4-5% zinc, 3-4% magnesium and about 0.4% chromium as well as varying amounts of copper, are considerably improved with respect to their resistance against stress corrosion by addition, in accordance with the present invention, of silver to the alloy, even if the copper content is kept extremely low.

Especially important is the surprising advantageous result obtained by adding, in accordance with the present invention, silver to alloys which due to their high content of magnesium and zinc possess very high strength properties in hot hardened condition (o- =65 kg./mm. a =6O kg./mm. 6=5%, wherein o-Bsigma B--indicates tensile strength, a0.2--sigma 0.2-yield strength, and 6-delta-elongation in percent of original length). The use of these high magnesium high zinc-content aluminum alloys was limited so far and connected with great uncertainties. Comparison of the results obtained with alloy 7 with the results obtained with alloys 8 and 9 which in accordance with the present invention contained both silver and chromium, clearly shows the vast superiority of the workpiece formed according to the present invention of aluminum, zinc, magnesium alloys containing both chromium and silver. As stated further above, the chromium in the alloy according to the present invention may be partially or fully replaced by manganese or vanadium. However, it is essential that silver and at least one of the three metals chromium, manganese or vanadium are included in the aluminum, zinc, magnesium, alloy in the indicated percentage range. It is essential in accordance with the present invention in order to obtain the novel and advantageous results thereof that the zinc and magnesium containing aluminum alloy which also may contain copper, always contains silver and at least one of the metals chromium, manganese and vanadium.

Workpieces or tools are produced according to the present invention by mechanically Working an aluminum alloy of the herein disclosed composition. Any of the conventional methods of working the alloy may hereby be employed, such as use of extrusion presses or die presses, forging, rolling, drawing or compressing.

The following examples are given as illustrative of the method of the present invention only, the present invention however not being limited to the specific details of the examples.

Example 1 An aluminum alloy having the following composition:

Percent Zinc 4.50 Magnesium 3.40 Chromium 0.20 Silver 0.66 Iron 0.28 Silicon 0.17 Aluminum Balance to 100 The 280 millimeter long billet sections were first heated in an annealing furnace to a temperature of 450 C., then introduced into the receiving container of the extrusion press and by means of a hydraulically activated upper die,

pressed through the opening of the die insert. The latter was so dimensioned as to form a T-profile of the dimen- 6 sions 60 x 40 x 8 x 4 millimeters. Thethus produced T-profile body was annealed for 15 minutes in a salt bath at 450 C., then quenched in water and finally age-hardened for 8 hours at C.

The resulting work pieces showed the following charactcristics:

Ultimate strength kg./mm. 55.7 Yield strength kg./mm. 46.9 Elongation percent 9.5

The work piece was then tested for stress corrosion resistance according to the bending method described further above. The work piece was still undamaged after a test lasting for 120 days and did not show any cracks.

was casted into billets of 300 millimeter diameter. After cutting down of the oxide skin, billets of 700 millimeter length were transformed on an extrusion press at a temperature of 430 C. into bars of flat section of 100 x 60 millimeters. Individual sections of these bars having a length of 600 millimeters were deformed in a hydraulic die press of conventional design at a temperature of 430 C. into pressed parts the center part of which corresponded to a T cross-section measuring x 90 x 25 x 15 mm. The solution heat treatment of these parts took place in a salt bath at 430 C. with subsequent quenching in water. The work pieces were then age-hardened for 8 hours at 110 C.

The thus produced work pieces showed a longitudinal direction of the fibers the following characteristics:

Ultimate strength; kg./mm. 67.8 Yield strength ..kg;/mm. 64.3 Elongation percent 5.5

The test for stress corrosion resistance showed results similar to those described in Example 1.

Example 3 Round billets having a diameter of millimeters and a length of 170 millimeters and consisting of an aluminum alloy of the following composition:

Percent Zinc 4.52 Magnesium 3.49 Manganese 0.91 Silver 0.82 Iron -1 0.22 Silicon 0.16 Aluminum Balance to 100 Ultimate strength kg./mm. 54.3 Yield strength kg/mm?" 45.8

Elongation "percent" 12 following dimensions:

"7 The'test for stress corrosion resistance showed results similar to those describedin Example' 1.

Example 4 An aluminum alloy A which did not contain any silver and analuminum alloyB which, in accordance with the present invention contained silver and chromium were similarly treated and the characteristics and resistance of the work pieces were compared COMPOSITION OF ALLOYS Alloys A and B were cast into rolling ingots of the 150 x 450 x 1100 mm. After cuttingdown the surface oxide skin, the rolling ingots were hot rolled at 450 C. to 6 millimeter thickness,

thereafter divided and cold rolled to 2 millimeter thickness. The thus obtained sheets were annealed for 30 minutes at 475 C., thereafter quenched in water and finally age-hardened for 24 hours at 120 C. The thus .finished sheets showed the following characteristics:

Samples of both sheets were then immersed for 100 daysin a solution containing 3% sodium chloride and 0.1% hydrogen peroxide. Thereafter the sheets showed the following characteristics:

Sheet A Sheet B (without (with silver) silver) Ultimate Strength kg./mm. 31 42 Yield Strength kg./mn1. 39 48 Elongation .percent. 10

These results clearly indicate the superiority of alloy 3 over alloy A.

Thefollowing Table IV lists 10 additional examples numbered through 14 of alloy compositions according .to the present invention.

TABLE IV Alloy Zn, Mg, Cr, Mn, V, Ag, Fe, Si, Al,

No. perperperperperperperperpercent cent cent cent cent cent cent cent cent 3.43 0.27 0.05 0.29 0.14 Balance. 3.51 0.17 0.34 0.55 0.31 0.19 Do. 3.28 0 14 0.23 0 04 0.52 0.27 0.17 Do. 3.38 0.92 0. 0.25 0.12 Do. 3.68 0.15 0 05 '0. 71 0.21 0.11 Do. 2.10 0.33 0.41 0. 25 010 Do. 2.00 0.16 0.28 0. 47 0. 23 0.18 Do. 1. 85 0.19 0. 18 0. 51 0. 26 0.13 Do. 2.22 0.77 0.83 0.25 0.17 Do. 2.03 0.12 0.75 0.24 0.13 Do.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute :essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be secured by Letters Patent is:

1. An aluminum alloy consisting essentially of commercial aluminum and of between 4% and 12% zinc, between 1.5% and 6% magnesium the combined quantity ofzinc and magnesium amounting to at'least 7.4%, between 0.1% and 4% silver and copper of which between 0.l% and 1% are silver, andan amount of at least one additional metal being selected from the group consisting of between 0.1% and 0.6% chromium, between 0.l% and 1.5% manganese, and between 0.03% and 0.15% vanadium.

2. A hardened, shaped workpiece the working part of which is an aluminum alloy of high stress corrosion resistance consisting essentially of commercial aluminum and of between 4% and 12% zinc, between 1.5% and 6% magnesium the combined quantity of zinc and magnesium amounting to at least 7.4%, between 0.1% and 1.0% silver, and an amount of at least one additional metal being selected from the group consisting of between 0.1% and 0.6% chromium, between 0.1% and 1.5% manganese, and between 0.03% and 0.15% vanadium.

3. An aluminum alloy consisting essentially of commercial aluminum and of between 4% and 12% zinc, between 1.5 and 6% magnesium the combined quantity of zinc and magnesium amounting to at least 7.4%, between 0.l% and'l.0% silver, between 0.01% and 3.00% copper, and an amount of at least. one additional metal being selected from the group consisting of between 0.1% and 0.6% chromium, between 0.1% and 1.5% manga nese, and between 0.03% and 0.15% vanadium.

4. An aluminum alloy consisting essentially of commercial aluminum and of between 4% and 12% zinc, between 1.5 and 6% magnesium the combined quantity of zinc and magnesium amounting to at least 7.4%, between 0.1% and 1.0% silver, approximately 1.5% copper, and an amount of at least one additional metal being selected from the group consisting of between 0.1% and 0.6% chromium, between 0.1% and 1.5% manganese, and between 0.03% and 0.15% vanadium.

5. An aluminum alloy consisting essentially of commercial aluminum and of between 4%and 10% zinc, between 2% and 4% magnesium the combined quantity of zinc and magnesium amounting to at least 7.4%, between 0.15% and 1.0% silver,approximately 0.3% chromium, and between 0.01% and 1.5 copper.

6. An aluminum alloy consisting essentially of commercial aluminum and of between 4% and-10%zinc, between 2% and-4% magnesium the combined quantity of zinc and magnesium amounting to at least7.4%, between 0.15% and 1.0% silver, approximately 0.8% manganese and between 0.01% and 1.5% copper.

7. A hardened shaped toolthe working part of which is a mechanically worked aluminum alloy of high stress corrosion resistance consisting essentially of commercial aluminum and of between 4% and 12% zinc, between 1.5% and 6% magnesium the combined quantity of zinc and magnesium amounting to at least 7.4% between 0.1% and 1.0% silver, and an amount of at leastone additional metal being selected from thegroup consisting of between 0.1% and 0.6% chromium, between 0.1% and 1.5% manganese, and between 0.03% and'0.15% vanadium.

8. A workpiece having high stress corrosionv resistance, said workpiece being made by mechanically Working an aluminum alloy consisting essentially of commercial 10 aluminum and of between 4% and 12% zinc, between References Cited in the file of this patent 1.5 and 6% magnesium the combined quantity of zinc UNITED STATES PATENTS and magnesium amounting to at least 7.4%, between 0.1% and 1.0% silver, and an amount of at least one 1,899,465 Kamps 1933 additional metal being selected from the group consist 5 2,261,210 Beck et a1 4, 1941 ing of between 0.1% and 0.6% chromium, between 0.1% 2,290,026 Bonsack July 1942 and 1.5% manganese, and between 0.03% and 0.15% FOREIGN PATENTS Vanadmm' 656,476 Great Britain Aug. 22, 1951 

1. AN ALUMINUM ALLOY CONSISTING ESSENTIALLY OF COMMERCIAL ALUMINUM AND OF BETWEEN 4% AND 12% ZINC, BETWEEN 1.5% AND 6% MAGNESIUM THE COMBINED QUANTITY OF ZINC AND MAGNESIUM AMOUNTING TO AT LEAST 7.4%, BETWEEN 0.1% AND 4% SILVER AND COPPER OF WHICH BETWEEN 0.1% AND 1% ARE SILVER, AND AN AMOUNT OF AT LEAST ONE ADDITIONAL METAL BEING SELECTED FROM THE GROUP CONSISTING OF BETWEEN 0.1% AND 0.6% CHROMIUM, BETWEEN 0.1% AND 1.5% MANGANESE, AND BETWEEN 0.03% AND 0.15% VANADIUM. 